A method of measuring current in switched circuits, a circuit for measuring current, and use of the method and the circuit.
The invention relates to a method of measuring current in a switching circuit of the two-port type whose first set of terminals is connected to a set of terminals of a noise reducing circuit , wherein a second set of terminals of the switching circuit has a switching terminal, which is adapted to be switched between the set of terminals of the noise reducing circuit.
The invention moreover relates to a circuit for measuring current in a switching circuit of the two-port type whose first set of terminals is connected to a set of terminals of a noise reducing circuit, wherein the switching circuit has a second set of terminals having a switching terminal capable of being switched between the second set of terminals of the noise reducing circuit.
The invention finally relates to use of the method and the circuit.
Traditional current measurements in switched amplifiers require a resistor in the signal path to the load or to the power supply of the amplifier. Use of such a measuring resistor in a switched amplifier, consisting of a multiple of half bridges, used for audio amplification results in poorer audio performance and a lower overall efficiency of the amplifier. Furthermore the voltage measured over the resistor must be very small to obtain a low power loss, but the very low voltage yields a need of an measurement amplifier, and thereby a higher complexity of the audio amplifier, to amplify the signal before it can be used in standard audio amplifier circuits where circuit voltages up to 5 Volts are common. It is also known that switched amplifiers need a very fast reacting current protection system to prevent the amplifier from damage if the output is short circuited. This means that the current measuring circuit must be very fast i.e. have a high bandwidth. If the current measuring circuit is based on a measuring resistor and a measuring amplifier with a high gain it is known from earlier designs that it is a complicated task to realise a high bandwidth of the circuit at the same time.
On the other hand switched amplifiers need to be protected against destructive high and fast rising currents for example due to short circuiting of the amplifiers output. Therefore it has long been attempted to find a principle to measure the current which allows the current to be measured without the above mentioned drawbacks.
The principle of a switched amplifier is that two or more switches in a half-bridge or a multiple thereof is switched to conduct and non-conduct, respectively, for a period of time, depending on the amplitude of a signal, such as an audio signal, fed to the amplifier. The information in the audio signal is hereby converted into a plurality of pulses (the signal is pulse-modulated) which carefully correspond to the useful information of the audio signal. To ensure a good linearity of the switched amplifier the pulses need to be very well defined.
Pulse-modulated amplifiers are in theory very linear and have low power losses, with a low distortion and an efficiency of 100%, but it has been found in practice that unlinearities and much lower efficiencies have caused them to be unsuitable for use in High Fidelity amplifiers, the reason being primarily that it is not practically possible to provide ideal pulses, high efficiency and high reliability of the amplifier at the same time.
One of the reasons why the pulses cannot be generated ideally is to be found in the power supply of the switches. The pulse heights vary as a consequence of variations in the power supply voltage, and therefore it is desirable to have a very well defined power supply voltage, but it is not possible to obtain a well defined power supply voltage with a measuring resistor inserted between the power supply and the switched amplifier output stage, due to the varying voltage over the measuring resistor when current is drawn.
It is more over seen that if a measuring resistor is inserted between the output stage and the load, the output impedance of the amplifier will rise. In amplifiers with feedback it is possible to decrease the output impedance of the amplifier by increasing the feedback, but in open loop or limited feedbacked amplifiers like true digital amplifiers and switched amplifiers in general this is not obtainable.
Furthermore it is seen that the efficiency of the amplifier will decrease if a resistor is inserted in the signal path either if it is between the output stage and the power supply or the load, due to current losses in the resistor. In prior used amplifiers of class A and AB types, this has been accepted as a minor source to the losses, due the low efficiency of the amplifiers output stages. But in switched amplifiers where extremely high efficiencies are obtainable the power losses in a measuring resistor can easily increase the total power losses of the amplifier with more than 50%.
In prior art there has been put a lot of effort in reducing the current-losses in the measuring resistance, among the presented solutions, a resistor with a very low resistance has been used, this of course results in a low voltage over the measuring resistance that needs to be amplified by a high gained measuring amplifier before it can be used in circuits operating at voltage levels of up to 5 volts. High gained amplification of very small voltages can be obtained, with limited bandwidths, in low noise circuits like class A and class AB amplifiers, but it is very difficult to obtain even a limited bandwidth amplified signal in the environment of a switched amplifier, due to noise generated by very fast slew rates on voltage and current sources in the amplifier. As mentioned earlier there is a need for fast reacting current protection systems in switched amplifiers due to the nature of the output stage therefore it has been seen in earlier designs that the current measuring circuits have been realised with a measuring resistor with higher resistance and thereby higher losses.
Accordingly, an object of the invention is to provide a method which is capable of measuring the current in a switched amplifier output stage and provide the measured signal with a high bandwidth at the desired voltage level, without the use of a measuring resistor in the signal path of the amplifier and without the need of an amplifier circuit to amplify the measured signal.
The object of the invention is achieved by the method defined in the introductory portion of claim 1, which is characterised in in that the current measurement is performed as a transient measurement during the switching sequences in the switching circuit, and that the measurement is performed across an impedance in the noise reducing circuit, said impedance being provided in parallel with the switching circuit.
A circuit is hereby provided which makes it possible to obtain a useful current sensor signal at the desired voltage level, without adding a loss and distortion generating current measuring resistor either in the output terminal or in the power supply terminal of the switched circuit.
Expedient embodiments of the method are defined in claims 2 and 3.
As mentioned, the invention also relates to a circuit of the type defined in the introductory portion of claim 4.
This circuit is characterized in that the current in the switching circuit during the switching sequence is caused to run in a measuring impedance circuit in the noise reducing circuit.
Expediently, as stated in claim 5, that the measuring impedance circuit is formed by an inductance which is connected between a terminal of the the first set of terminals of the switching circuit and a terminal of an input port of the noise reducing circuit, and in that a series connection of a capacitor and a measuring resistor is connected in parallel with the first set of terminals of the switching circuit.
When, as stated in claim 9 that the measured current is fed to a control circuit which is adapted to instantaneously switch off the switching terminal of the switching circuit, the additional effect is achieved that the control circuit can instantaneously disconnect the switches at a predetermined current value, so that these are protected against destruction by excess current.
Other expedient embodiments of the circuit are defined in the dependent claims.
Finally, as mentioned, the invention relates to use of the method and the circuit.
This use is defined in claim 12.
In this use it is possible to provide a low-noise half-bridge for audio use which has a low pulse distortion, as well as a much simpler and quicker protection of the switches than traditional protecting circuits for half-bridges.
It should be noted that the principles of the invention are extremely suitable for use in connection with a noise attenuating circuit, which is described more fully in WO 99/59241 filed with the same priority as the present application.